Oomycetes
The Oomycota are filamentous protists which gain their nutrition by absorbing food from surrounding water or soil, or by invading the body of another organism to feed on fluids there. There are more than 500 species in the Oomycota, including the so-called water molds and downy mildews. As such, oomycetes play an important role in the decomposition and recycling of decaying matter. Parasitic species have had a impact on human activities by destroying crops or fish.
“Oomycota” means “egg fungi,” which refers to the large round oogonia, or structures containing the female gametes. Oomycetes are oogamous, producing large non-motile gametes called eggs, and smaller gametes called sperm. The Oomycota have a very sparse fossil record. A possible oomycete has been described from Cretaceous amber.
The Oomycota were once classified as fungi, because of their filamentous growth, and because they feed on decaying matter as do fungi. The cell wall of oomycetes is not, however, composed of chitin as in the fungi, but instead is made up of a mix of cellulosic compounds and glycan. Another distinguishing feature is that the nuclei within the filaments are diploid, with two sets of genetic information, not haploid as in the fungi.
The ultrastructure, biochemistry, and molecular sequences of these organisms indicate that they belong with the Chromista. The free-swimming spores which are produced bear two dissimilar flagella, one with mastigonemes, a feature that is common in the chromists, as is the presence of the chemical mycolaminarin, an energy storage molecule similar to those found in kelps and diatoms. Thus, oomycetes are classified as belonging to the taxonomic minority known as heterotrophic chromists.
Some oomycetes, also called molds or water molds, are parasites on other organisms. Water molds may grow on the scales or eggs of fish, or on the skins of amphibians. The water mold Saprolegnia causes lesions on fish which cause problems when the water is stagnant as is often the case in aquaria or fish farms, or at high population densities such as when salmon swim upstream to spawn. Saprolegnia can spread rapidly, damaging a large surface area. These infections can be difficult to treat. Other species of Saprolegnia are parasitic on aquatic invertebrates such as rotifers, nematodes, and arthropods, and on diatoms.
Their greatest impact on humans, however, comes from the many species of oomycete which are parasites on flowering plants. These include root rotting oomycetes, seedling dampening mold, blister rusts, white rusts Albugo, and the downy mildews that affect grapes, lettuce, corn, cabbage, and many other crop plants. Two of these disease-causing oomycetes have had a major impact on world history.
The first of these is Phytophthora infestans, the organism which causes late blight of potato. The potato is native to South America, but after it was introduced to Europe in the late 16th century, it quickly became an important food crop. Late blight did not follow its host plant across the Atlantic until much later. The disease organism grows into the stem and leaf tissues, causing death, and may also infest the tubers. The disease spreads rapidly under cool and damp conditions, which are common in western Europe. In one famous case, in just one week during the summer of 1846, this disease wiped out almost the entire potato crop of Ireland, where potatoes were the primary food of the poor. This Phytophthora blight caused the deaths of nearly a million inhabitants of Ireland, and precipitated the emigration of an additional 1.5 million to other countries. Other species of Phytophthora destroy eucalyptus, avocado, pineapples, and other tropical crop plants. While chemicals have been developed to combat oomycete infections, the emergence of chemical-resistant strains combined with banning of effective chemicals has combined to create a P. infestans epidemic which is now a serious problem.
The other oomycete which has severely impacted recent history is Plasmopara viticola, the downy mildew of grapes. It is a native of North America, but in the late 1870s was accidentally introduced to Europe. At the time, the French wine industry was concerned over a massive aphid infestation, and so brought resistant vine strains over from America to breed them into their own grapes. When these American stocks arrived, the American vines also brought the downy mildew, which almost wiped out the entire French wine industry. The industry was saved by the serendipitous discovery of Bordeaux mixture, a mixture of lime and copper sulfate, which brought the disease under control when applied to the leaves of the plants. This discovery is also important for being the first known fungicide, and in fact the first chemical used to control a plant disease. However, Bordeaux mixture is hazardous to many other organisms.
A current problem is Sudden Oak Death Syndrome which is caused by a previously unknown species of Phytophthora. First observed in 1995, within 5 years the infestation by this plant pest has spread 350 miles along the California coast infecting tan oaks, coast live oaks and black oaks. In some areas, as many as 80% of the trees are infected. A state of emergency has been declared in Marin County, one of the hardest-hit areas. Effective, environmentally-safe means to combat this Phytophthora species have not been determined.
This disease not only impacts oaks, but also the thousands of animal species that rely on leaves and acorns from these trees, as well as increasing the fire risk posed by the rapid accumulation of dead trees.
FtsZ proteins
FtsZ (named after filamenting temperature sensitive strain Z) is a 40 kDa protein ubiquitous in Eubacteria and Archaea. The bacterial cell division protein FtsZ is a key component of the bacterial cell division machinery. Fusion constructs of FtsZ with green-fluorescent protein have shown that, at the onset of division, FtsZ forms a filamentous ring at the site of cell division, and disassembles after septation is complete. FtsZ can self-assemble into rafts of long filaments having curving edges, as well as into sheets and rings. A cytoskeletal role for FtsZ has been postulated based on its ability to undergo GTP-dependent polymerization in vitro and its similarity to tubulin. Bacterial FtsZ shares limited sequence identity with tubulin, and the axial repeat of these filaments is around 40 Å, the same as that of tubulin monomers in a protofilament. The structure of FtsZ has been solved by X-ray crystallography using crystals obtained from the FtsZ1 protein from the hyperthermophilic methanogen Methanococcus jannaschii. The model, refined to 2.8 Å, includes a molecule of GDP.
Until recently, the only known eukaryotic FtsZs were chloroplastic FtsZs (FtsZ-cp) in plants. In higher plants, the FtsZ protein is involved in plastid division, but there is little information on its involvement in the plastid-dividing apparatus. Comparison of several prokaryotic and eukaryotic FtsZ proteins shows that there are six highly conserved domains in the core region of FtsZ. Phylogenetic analysis indicates that Cyanidium caldarium RK-1 and other eukaryotic FtsZ genes are the descendants of cyanobacterial FtsZ genes, supporting the current agreement that FtsZ is involved in plastid division. Expression studies of the gene encoding FtsZ (the FtsZ gene) in C. caldaium indicated that the FtsZ gene is transcribed just before plastid division. Eukaryotic FtsZ isolated from Arabidopsis thaliana contains a glycine-rich tubulin signature motif which is conserved among FtsZ proteins and tubulins, and which is important for GTP binding, which further supports the suggestion that eukaryotic FtsZ proteins may have a cytoskeletal role analogous to that of tubulin. (U.S. Pat. No. 5,981,836).
In major groups of eukaryotes, such as animals, plants and true fungi, mitochondrial division is mediated by a non-FtsZ mechanism. There are no FtsZ genes in yeast or nematode where the respective genomes have been completely sequenced. The recent discovery of the mitochondrial form of FtsZ (FtsZ-mt) in a chromophyte alga (Beech et al. (2000) Science 287: 1276-1279) strongly suggests that in primitive eukaryotes, unlike major groups of eukaryotes, FtsZ-mt is required for mitochondrial division.